In recent years, the amount of information handled by computers has been increased and consequently an increase in the capacity of a hard disk unit as an auxiliary memory apparatus has been demanded. Further, along with progress for the mounting of a hard disk unit to domestic electric products, demand for decreasing the size and increasing the capacity of the hard disk unit has been increased more and more. While a hard disk unit using a longitudinal magnetic recording system has attained an in-plane recording density exceeding 20 Gbits/cm2, it has become difficult to further increase the recording density by using the system. As an alternative system, a perpendicular recording system has now been investigated. It is considered that the perpendicular recording system is more advantageous compared with the longitudinal magnetic recording system for increasing density since it suffers less from effects of the demagnetizing field in a high density recording region.
For the perpendicular magnetic recording medium used for the perpendicular recording system, a recording layer comprising a CoCrPt alloy used in the longitudinal magnetic recording medium has been considered. However, for further decreasing noise, a granular type recording layer with the addition of oxygen or oxide to the CoCrPt alloy has been proposed and has attracted attention. The granular type recording layer is described, for example, in Japanese Laid-Open Patent No. 2001-222809A and Japanese Laid-Open Patent No. 2003-178413A. In a case of a conventional recording layer comprising the CoCrPt alloy, a non-magnetic material mainly comprising Cr is segregated to the grain boundary by utilizing the phase separation between Co and Cr. Therefore, the magnetic crystal grains are isolated magnetically, thereby reducing the noise. While it is necessary to add more Cr in order to increase the effect of noise reduction further, Cr remains in a great amount also in the magnetic crystal grains. In this case, it results in a problem of lowering the magnetic anisotropy energy and deteriorating the stability of recording signals. On the other hand, in a case of a granular type recording layer in which oxygen or oxide is added to the CoCrPt alloy, since the oxide is separated easily from magnetic crystal grains, a structure where the oxide surrounds the magnetic crystal grain can be formed without addition of a large amount of Cr so long as a template trigger of forming an oxide grain boundary is formed in an underlayer. Further, since the amount of Cr contained in the magnetic crystal grain can be decreased, noises can be decreased without lowering the magnetic anisotropy energy (Ku). However, while the medium described above has large Ku and excellent thermal stability, it causes deterioration of the overwrite characteristic (hereinafter referred to as OW characteristic) since the coercivity of the medium increases. For improving the OW characteristic, there is a method of increasing the Cr addition amount in the recording layer or a method of decreasing the thickness of the recording layer, but neither is preferred since the thermal stability is degraded. As a method for solving the problem, a perpendicular magnetic recording medium in which a recording layer comprising a CoCr alloy without an oxide is laminated over a granular type recording layer where an oxide is added to a CoCrPt alloy has been proposed as describe, for example, in Japanese Laid-Open Patent No. 2004-310910A and in IEEE Transactions on Magnetics, Vol. 41, No. 10, p. 3145 (2005). It has been reported that the perpendicular magnetic recording medium of such a laminate type recording layer is excellent in the thermal stability and can provide a sufficient OW characteristic and high medium SNR compared with the conventional perpendicular magnetic recording medium having a single layered granular type recording layer.
For attaining a high density recording exceeding 30 Gbits/cm2, it is necessary to increase the track density together with the linear recording density and, for improving the track density, it is necessary to make the track width of the magnetic head finer and enhance the accuracy. In addition, as the track becomes narrower, a problem occurs when the effective track width to be written into a medium is larger than the geometrical track width, which forms a bottleneck for the improvement of the track density. For solving this problem, a shielded pole writer has been developed as shown, for example, in U.S. Patent Publication No. 2002/0176214A1 or Japanese Laid-Open Patent No. 2005-190518A. The shielded pole writer is an improved single-pole-type writer where a write section includes a main pole and an auxiliary pole disposed on the leading side in the down-track direction, which is a head of a structure in which a magnetic shield is disposed so as to cover at least the trailing side in the down-track direction of the main pole via a non-magnetic gap layer for improving write-field gradients. Hereinafter, the shield means a shield disposed near the main pole of the recording unit of the magnetic head.
Since the shielded pole writer has a higher effect of narrowing an effective track width to be written onto a medium, as well as it can increase write-field gradients compared with a conventional single-pole-type writer without shield, it has an effect of improving the medium SNR. However, since the write-field intensity is small, no sufficient OW characteristic can be obtained easily. That is, in a combination with a shielded pole writer capable of attaining high track density, there exists a subject in the medium design capable of obtaining high medium SNR while ensuring sufficient OW characteristic. As described above, while a perpendicular magnetic recording medium in which a recording layer comprising a CoCr alloy not containing oxide is stacked on a granular type recording layer with addition of an oxide to a CoCrPt alloy is excellent in the OW characteristic, the condition for obtaining a sufficient OW characteristic is still severe even in the case of using such a medium when it is in combination with the shielded pole writer and the medium SNR is inevitably degraded. That is, it has been extremely difficult to attain a recording density exceeding 30 Gbits/cm2 by overcoming a trade off relation between the OW characteristic and the medium SNR.